1. Field of the Invention
The present invention relates to disc drives for storing retrievable informational data on a rotary medium where, in response to a common data track demand signal both a transducer is positionable to interact with selectable radii on the medium and the angular velocity of the medium is controllable to maintain the surface velocity of the medium relative to the transducer substantially constant regardlessly of the radius of the transducer on the medium.
2. The Prior Art
It is well known to record informational data in a plurality of concentric, circular data storage tracks on a rotating disc, where the disc rotates with a constant angular velocity and where the tracks are accessed by the placing in adjacence thereto of a reading or writing head.
The head has a lower limit of resolution. If two consecutive recorded features are less than a predetermined distance apart the head cannot properly distinguish between them. If the amount of data on the disc is to be maximised the data on all tracks must approach as nearly as possible to maximum density where the limit of resolution is achieved. Data is generally recorded in response to the regulation of a fixed-frequency crystal oscillator clock. The angular velocity of the medium being constant, the surface velocity of the medium beneath the head is least at the least recording radius on the disc and the density of stored binary digits is greatest for the smallest-radius track. The number of bits on the other tracks is constrained to be the same for each of them as for the least radius track. If the innermost track is at maximum data density, the others are not since they are longer and the storage capacity of the disc is under-utilised.
In order to overcome this difficulty a first style of solution was adopted where the data were recorded and recovered from the surface of the disc at different frequencies dependently upon the radius of the track whereon they were recorded. This was known as "zoning" and had the considerable disadvantage that large amounts of additional clock and decoding hardware were required in order for the system to operate. In those systems where the ratio of the maximum to minimum recording radii on the medium was small the percentage gain in data capacity was not worth the expense of attainment and in those systems where the ratio was large the additional cost of the numerous additional components rendered the method unattractive despite the potentially large storage capacity improvement.
Another style of solution is exemplified by U.S. Pat. No. 3,826,965 granted July 30, 1974 for an invention by Charles Brignol. Brignol discloses a system where the surface velocity of a disc beneath a transducer is maintained substantially constant by a linear potentiometer affixed so that its wiper is co-movable with the head. The two ends of the potentiometer and the wiper are taken as input to a hyperbolic function generator which generates output inversely proportional to the radius of the head over the disc. The output of the function generator is used to control the speed of the motor which turns the disc so that the angular velocity of the disc is inversely proportional to the radius of the head. This disclosure can be taken as representative of all methods where a transducer senses the radius of the head and the output of the transducer is coupled to control the angular velocity of the disc. All such methods have the considerable disadvantage that the speed-controlling demand signal to whatever system is controlling the angular velocity of the disc must wait to change as the radius of the head changes. The demand signal must always change in sympathy with the position of the head and in consequence the control mechanism for the disc angular velocity must lag behind the position of the head. Thus, if the head is sent from an outer to an inner track, the head will arrive at its destination radius before the speed of the disc has stabilised at its new value, since the disc angular velocity control system did not know the final value of demand signal until the head actually arrived. It is important in disc drives that data decoding can begin as soon as possible after the arrival of the head over its destination track. Correct data decoding requires that the angular velocity of the disc be substantially correct and having to wait for the disc speed to stabilise is not desirable.